In the industrial production of solar cells, monocrystalline or polycrystalline silicon wafers are cut from massive ingots mainly by sawing. This creates a rough surface having a mean surface roughness of about 20 to 30 μm, customarily referred to in the art as saw damage. This saw damage is usually caused by metal attrition of the sawing wire and residual abrasives. It is therefore necessary to carry out a so-called saw damage etch to remove the surface roughness and to texturize the silicon wafer surface. This way, a certain roughness is created at the surface which roughness enables the multiple reflection of light incident on the surface, thereby leading to greater absorption of the light inside the silicon wafer, i.e., to an increased light-confining effect.
Following the texturization, short treatments of the textured wafers with either water or alkaline or acidic solutions can be carried out. Alternatively or additionally, a conventional finishing by a short treatment with a hydrogen fluoride containing solution can be carried out. The hydrogen fluoride removes the natural oxide layer at the surface of the silicon wafers accompanied by the formation of silicium-fluorine bonds. This way, an activated hydrophobic silicon surface is created.
The silicon tetrafluoride which is generated as an intermediary by the hydrofluoric acid treatment can react with water to produce colloidal silicon dioxide particles which tend to adhere to the activated silicon surface and may form spots or stains called “haze”. Additionally, due to the surface tension of the water, the hydrophobicity of the surface leads to the formation of water droplets during the rinsing step. The colloidal particles however tend to concentrate on the vapor-liquid boundaries of the droplets. During the drying step the droplets can roll along the silicon wafer surface such that the colloidal particles contained in the droplets adhere to and re-contaminate the silicon wafer surface.
Moreover, the hydrophobic silicon wafer surface can hardly be wetted by highly polar spray-on phosphorus emitter sources such as aqueous or alcoholic phosphoric acid. Therefore, the silicon wafer surfaces have to be rendered hydrophilic before they can be contacted with the phosphorus emitter source.
Many aqueous alkaline etching and cleaning compositions for the treatment of the surface of silicon wafers have been proposed in the prior art.
Thus, already the Japanese patent application JP 50-158281 discloses the use of an aqueous solution of tetramethylammonium hydroxide (TMAH) and hydrogen peroxide for the cleaning of semiconductor wafer surfaces.
The American U.S. Pat. No. 4,239,661 proposes the use of an aqueous solution containing choline and hydrogen peroxide and additionally containing nonionic surfactants such as aliphatic esters of polyhydric alcohols or polyethyleneoxides, complexing agents such as cyanide or ethylenediaminetetraacetic acid (EDTA), triethanolamine, ethylenediamine or cuproin, for treating and washing of the surface of intermediate semiconductor products, the etching of metal layers and the removal of positive-working photoresists.
The German patent application DE 27 49 636 discloses the use of an aqueous composition containing TMAH, hydrogen peroxide, complexing agents such as ammonium hydroxide or pyrocathechol, fluorinated compounds as surfactants such as hexafluoroisopropanol, and inhibitors such as ammonium fluoride, ammonium biphosphate or oxygen.
The Japanese patent application JP 63-048830 discloses the removal of metal impurities from silicon substrate surfaces after a hydrofluoric acid treatment with an aqueous composition containing choline and hydrogen peroxide.
The Japanese patent application JP 63-274149 discloses the degreasing and the removal of inorganic contaminants from semiconductor wafer surfaces with aqueous compositions containing TMAH, hydrogen peroxide and nonionic surfactants.
The American U.S. Pat. No. 5,129,955 describes the cleaning and the hydrophilization of silicon wafer surfaces after the hydrofluoric acid treatment with an aqueous solution of choline or TMAH and hydrogen peroxide.
Likewise, the American U.S. Pat. No. 5,207,866 discloses the use of such compositions for the anisotropic etching of monocrystalline silicon.
The European patent application EP 0 496 602 A2 describes the removal of metal impurities from silicon wafers surfaces with aqueous compositions containing TMAH, hydrogen peroxide and complexing agents such as phosphonic acids or polyphosphoric acids.
The American U.S. Pat. No. 5,705,089 describes the removal of metal impurities from silicon wafers with aqueous compositions containing TMAH, hydrogen peroxide, complexing agents such as polyphosphonic acids, wetting agents such as polyhydric alcohols and anionic, cationic, nonionic and fluorinated surfactants, water-soluble organic additives such as alcohols, glycols, carboxylic acids, hydroxycarboxylic acids, polycarboxylic acids and polyhydric alcohols which may also be oxidized.
The European patent application EP 0 665 582 A2 proposes aqueous compositions containing TMAH, hydrogen peroxide and complexing agents having at least three N-hydroxylaminocarbamoyl groups as surface treating compositions for semiconductors and for the removal of metal ions.
The American U.S. Pat. No. 5,466,389 discloses the cleaning of silicon wafers leading to a reduced surface micro-roughness with aqueous compositions containing TMAH, hydrogen peroxide, nonionic surfactants, complexing agents and buffering components such as inorganic mineral acids and their salts, ammonium salts, weak organic acids and their salts and weak acids and their conjugate bases.
The American U.S. Pat. No. 5,498,293 proposes for this purpose aqueous compositions containing TMAH, hydrogen peroxide, amphoteric surfactants such as betaines, sulfobetaines, aminocarboxylic acid derivatives, iminodiacids, amine oxides, fluoroalkyl sulfonates or fluorinated alkyl amphoterics, complexing agents, and a propylene glycol ether solvent.
The American U.S. Pat. No. 6,465,403 B1 discloses alkaline cleaning and stripping compositions containing TMAH, hydrogen peroxide, quaternary ammonium silicates, complexing agents, water-soluble organic solvents, and amphoteric, nonionic, anionic or cationic surfactants.
The American U.S. Pat. No. 6,585,825 B1 discloses similar compositions additionally containing bath stabilizing agents such as weakly acidic or basic compounds, e.g., salicylic acid.
The American U.S. Pat. No. 6,417,147 describes cleaning compositions for removing contamination from the surface of semiconductor wafers, the compositions containing TMAH, hydrogen peroxide, fluorine containing anionic surfactants such as fluorinated alkenyl sulfonic acids having at least 6 carbon atoms to the molecule, alkanolamines, and nonionic surfactants.
The international patent application WO 02/33033 A1 discloses cleaning compositions for semiconductor wafers having metal lines and vias, the said compositions containing TMAH, hydrogen peroxide, a bath stabilizing agent such as salicylic acid, water-soluble silicates, complexing agents, and organic solvents.
The American US 2006/0154839 A1 discloses the use of aqueous compositions containing TMAH, hydrogen peroxide and phosphite or hypophosphite as stripping and cleaning compositions primarily for ash residue removal.
The American US 2006/0226122 discloses aqueous etching compositions containing TMAH, hydrogen peroxide, and aromatic sulfonic acids such as benzyl sulfonic acid. The compositions are primarily used for the selective wet etching of metal nitrides.
The American US 2010/0319735 A1 discloses cleaning compositions which are capable of removing both organic soiling and particulate soiling adhered to a substrate for an electronic device. The cleaning compositions contain a water-soluble salt containing a transition metal, a chelating agent and a peroxide. Additionally, the cleaning compositions can contain alkali agents such as ammonia, tetramethylammonium hydroxide and tetraethylammonium hydroxide, anionic surfactants such as linear alkyl benzenesulfonates, alkyl sulfates and alkylether sulfates, and nonionic surfactants such as alkyleneoxide adducts of higher alcohols.
However, the hydrophilizing effect of these prior art etching and cleaning compositions needs considerable improvement in order to be able to meet the increasingly stricter demands of the modern processes for manufacturing highly efficient solar cells.
In particular, the unsatisfactory hydrophilicity of the surfaces of the silicon substrates, especially, of the surface of silicon wafer surfaces, makes it difficult to evenly distribute highly polar spray-on phosphorus sources which, in turn, leads to an unsatisfactory phosphorus doping and, consequently, to solar cells having an unacceptably low efficiency.
After the removal of the etching and cleaning compositions, phosphorus emitter sources can be applied single-sided or double-sided onto the silicon wafer surfaces in the next process step. The applied phosphorus emitter sources are heated, for example, in an infrared-heated belt furnace so that the phosphorus emitter diffuses into the silicon substrate.
In this process step, a layer or zone of phosphorus silicate glass (PSG) and second zone the so-called dead layer, which consists of non-electrically active phosphorus, are formed on top of the surface of the silicon wafers.
However, whereas the layer PSG layer can be substantially removed by a hydrofluoric acid treatment in the next process step, this is not the case with the dead layer. The dead layer however impairs the electrical characteristics of the solar cells and particularly decreases the short-circuit current and thereby the efficiency.
In the art, gaseous phosphorus sources such as POCl3 can also be used for the generation of phosphorus emitters in the silicon substrate. In this case, no hydrophilizing step is required after the texturization of the silicon substrate. However, the problems associated with the dead layer remaining after the PSG layer removal still need to be remedied.
Moreover, the phosphorus emitter layer which is present on both sides and/or on the edges of the silicon substrate after the phosphorus doping must be isolated to prevent short-circuiting the solar cell. Edge isolation can be accomplished by laser edge isolation techniques after the metallization step or by wet chemical etching.
The wet chemical edge isolation is accomplished by immersing the rear side and the edges of the silicon substrate in a hydrogen fluoride containing composition. Due to surface tension effects between the substrate and the hydrogen fluoride containing composition, the emitter layer on the front side is not exposed to the etching. However, residues of porous silicon can remain which must be removed before the further processing of the silicon substrate.
Therefore, in modern process sequences for manufacturing devices generating electricity upon exposure to electromagnetic radiation, additional wet cleaning and surface modification steps followed by rinsing and drying are carried out after the PSG removal step and/or the wet edge isolation step and before a silicon nitride (SiNx:H) antireflection coating is applied, for example, by physically enhanced chemical vapor deposition (PECVD). By way of such an additional wet cleaning and surface modification step the debris which is left over from the PSG removal step and/or the wet edge isolation step and/or has re-contaminated the silicon wafer surface as well as the dead layer and/or porous silicon residues are removed and the surface is modified by etching and oxidation.
It would be highly desirable, both in economic and technical terms, if the etching and cleaning compositions used in the hydrophilizing step could also be used for the additional wet cleaning and surface modification steps. The prior art etching and cleaning compositions may be suitable for both purposes to a certain extent. However, further improvements are needed in order to meet the ever-increasing technical and economical demands of the solar cell manufacturers.